Detection of end zones in alternating track circuits

ABSTRACT

A continuous rail track circuit apparatus having termination shunts at each end detects the presence of a railroad vehicle shunt across the rails and indicates vehicle presence. Shunt means near one end of the track circuit periodically introduces a test shunt across the track, and receiver means coupled to the rails near the other end of the track circuit is sensitive to changes in track circuit parameters occasioned by the test shunt and respondingly introduces a reply test shunt in accordance with the period of the shunt means. Means at the first end of the circuit sensitive to changes in track circuit parameters occasioned by the reply test shunt provides indication of vehicle presence when the change in track circuit parameters ceases.

United States Patent Sibley [54] DETECTION OF END ZONES IN ALTERNATINGTRACK CIRCUITS [72] Inventor: Henry C. Sibley, Adams Basin, NY. [73]Assignee: General Signal Corporation, Rochester,

22 Filed: May 4,1910 21 Appl.No.: 34,237

[151 3,663,809 [4 1 May 16, 1972 Primary Examiner-Arthur L. La PointAssistant Examiner-George H. Libman Attorney-Harold S. Wynn 5 7]ABSTRACT A continuous rail track circuit apparatus having terminationshunts at each end detects the presence of a railroad vehicle shuntacross the rails and indicates vehicle presence. Shunt means near oneend of the track circuit periodically introduces a test shunt across thetrack, and receiver means cou- [52] U 8 Cl 246/34 CT 246/40 pled to therails near the other end of the track circuit is sensi- [511 23/30 tiveto changes in track circuit parameters occasioned by the test shunt andrespondingly introduces a reply test shunt in ac- [58] Field of Search..246/34 R, 34 CT, 36, 40 cordance with the period of the Shunt means-Means at the [56] References Cited first end of the circuit sensitive tochanges in track circuit parameters occasioned by the reply test shuntprovides indica- FOREXGN PATENTS OR APPUCATIQNS tion of vehicle presencewhen the change in track circuit parameters ceases. 1,036,142 9/1953France ..246/34 CT 9 Claims, 4 Drawing Figures T \1 END ZONE I CENTERZONE I END ZONE I T l PULSE lg GEN,

r l OFSC i l I R| f i i R2 a I9 56 28 l 2 I ""71 Pl F 3 J Rf DECODEF- ea Output DETECTION OF END ZONES IN ALTERNATING TRACK CIRCUITS BACKGROUNDOF INVENTION joints, two serious problems arise. First, the ends of thecircuit 1 are poorly defined so that the location of a vehicle is vague,and secondly, the signal is not confined to the circuit so that there isan interference along the various circuits along the track. Ifconductors cannot be interrupted, then the only way to terminate thecircuit is by a short. A track section terminated by shorting barsovercomes the two problems above reasonablywell, but adds a new problem.Since the train must be considered to be less than a perfect shortcircuit, the train close to the shorting bar will have very littleeffect on the track circuit and will not be detected. The length of thisblind spot depends on frequency and the adjustment of the receiver caneasily range between 2,000 feet at 65 hertz and feet at 7 kHz.

The scheme herein is for a track circuit which is terminated by ashorting bar and can detect a single 0.06 ohm shunt within a foot or twoof the ends.

It is therefore an object of the present invention to provide anarrangement which substantially obviates one or more of the limitationsand disadvantages of the described prior arrangements.

It is another object of the present invention to provide a continuousrail track circuit capable of accurately detecting a railroad vehiclenear the end zones.

SUMMARY OF INVENTION There has been provided a continuous rail trackcircuit apparatus having termination shunts at each end. The trackcircuit detects the presence of a railroad vehicle shunt across therails which is indicative of vehicle presence. Shunt means near one endof the circuit periodically introduces a test shunt across the tracksand a receiver means coupled to the rails near the other end of thecircuit is sensitive to changes in the circuit parameters occasioned bythe test shunt. The receiver periodically introduces a replytest shuntin accordance with the period of the shunt means and means at the firstend of the circuit is sensitive to changes in circuit parametersoccasioned by the reply test shunt and provides indication of vehiclepresence when the change in circuit parameters ceases.

The track circuit parameters referred to as being changed by the shuntare an increase in track circuit current in the area of a test shunt anda resulting increase in carrier current shift frequency feeding thetrack circuit as a means for communicating the detection of the testshunt at one end of the track circuit to the opposite end of the trackcircuit.

There has also been provided a track circuit having three zone definedtrack circuits including a center zone track circuit and two end zonetrack circuits having one end coupled to an end of the center zone andterminated at the other end of the termination shunt. The track circuitshave characteristics such that the signals present in the rails andblocked by a vehicle shunt present in the center zone and increased by avehicle shunt present in the end zones. Means responsive to both theblockage and increase in the signals provides indication of vehiclepresence.

For a better understanding of the present invention, togetherwith otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, whileits scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic diagram showing theapparatus necessary for practice of the present invention;

FIG. 2 is a circuit detail showing track coupling and the shunt means;

FIG. 3 is a repeater station for extending track circuit length; and

FIG. 4 is a waveform diagram showing introduction of coded informationas well as vehicle detection.

DESCRIPTION OF THE PREFERRED EMBODIMENT 0 These loops may be 8 feet longwith eight to 32 turns, the

larger number being required for low frequencies. Receivers sensecurrent at the ends opposite their transmitters by means of loops orsmall coils mounted close to the rail. The train anywhere along thetrack, except close to the ends, will reduce the current into thereceivers to the point where detection is certain. The ends are theproblem. A train close to the end will not reduce received currentappreciably, but it will increase the transmitted current. Receiverstuned to the local transmitter frequencies are added to each end todetect when track current is increased by a train.

The system described here shows three track circuits detecting vehicles.In the center zone or circuit most of the length of the track circuitvehicles are detected in the conventional closed circuit manner. The twoend zone circuits are checked by the open circuit series method and thethree readings are combined in a logic scheme that provides fail-safetrain detection, security against operation by false signals, and achoice of such options as a simple repeater to double the circuit lengthand message handling capability for traffic control and cab signals.

The end zone requiring special detection means is a length of trackhaving the shorting bar at one end and having an impedance ofapproximately 0.6 ohm looking into the other end. If a train having ashunt value of 0.06 ohm stands at the end of this track section, thecurrent divides sending approximately 90 percent through the train axleand leaving only 10 percent for the receiver at the far end of the tracksection. A train further from the end will cause a greater reduction inreceiver current because the end zone has even a higher impedance. Theseries circuits at the ends must detect a train at a distance where thetrack impedance is 0.6 of an ohm or more. These values of 0.6 ohm foratypical end zone impedance and 0.06 ohm for a standard shunt are set inaccordance with certain principles of safety for providing marginsrelative to the track conditions.

At a frequency of 500 hertz, a track circuit of this type will probablywork well over a mile with ballast resistances as low as 5 ohms in 1,000feet. The end zones; that is, the 0.6 ohm track sections are about 500feet long. A short poor shunting vehicle might be lost within 500 feetof the ends unless the special end zone circuits are used.

The transmitting loop and its 8 foot section of track may be consideredas a transformer feeding a mile long track circuit with a short at thefar end. This mile long track circuit will look like a load that variesbetween 2 and 7 ohms depending on ballast resistance if there are notrains. If a train is within 500 feet of the end, the transformer willfeed a load of 0.6 ohm or less. The receiver sensing transmitter currentmust indicate occupancy when the current is that value which fiowsthrough 0.6 ohm or less. At lower current values it need not indicateoccupancy because track current increases as the shunt moves away fromthe receiver and at the current value that flows into a mile of wettrack, for example, 2 ohms it must not indicate occupancy. There is arange, therefore, of current values which must satisfy an empty trackcircuit under worst ballast conditions; that is, a 2 ohm value for. theentire length and must be compatible with an arbitrarily set impedancevalue of 0.6 ohm for 500 feet.

The current detector is tested by simulating a train at the limit of theend zone. Since by the conventional method of track shunting it ispossible to detect a train at a distance of across the track at the endof the transmission loop and will affect the circuit such that the samecurrent in the track circuit is produced as if a train were presentwithin the 500 foot end zone section. This resistor is alternatelyswitched in and out of the circuit continually testing the end zonecircuit. This resistor affects only the end zone being tested and'hasnegligible effect on the signal transmitted from the end zone beingtested to the other end of the circuit. If this were not true, therewould be no problem detecting trains near the ends.

This track circuit uses track transmitters which are of the frequencyshift keyed type. Receivers responsive to the transmitters can detectwhich way the transmitters are shifted and have the added safety featurethat they will respond to both shifts simultaneously so that anyinterference that produces what looks like an impossible condition, willnot be resolved but will result in a safe failure. This carrierequipment plus cycle checked logic combine the end zone and main linesignals.

The following is a description of the overall scheme with respect toFIG. 1. The schematic diagram of FIG. 1 shows a railroad track circuitincluding a set of rails indicated by reference R terminated at end by atermination shunt S The first or left end of the track circuit is fed bya track transmitter T, including an oscillator capable of generating anF, frequency and an amplifier 11 which increases the gain of the carrierfrequency F, and couples it to the rails R through a loop transmitter12. A pulse generator 13 drives a track test shunt 14. Shown in FIG. 2,this consists of a relay l5 driving its contact 16 in series with a 0.6ohm resistor 17 coupled across the track rails R. This track test shunt14 periodically is coupled across the rails R and the shunt in the railsproduced by the coupling of the F, signal by loop 12 increases. Areceiver R1 responsive to the F, frequency is capable of detecting anincrease in track current. The receiver R, consists of a filter l8 and adetector 19 calibrated so as to detect some threshold current. When sucha condition exists as when the test shunt 14 is coupled across the railsR, the detector 19 provides a signal to the transmitter T, and inparticular to the oscillator 10. This signal causes the oscillator toshift up the basic frequency F, to some preselected value F This signalF,+ is thence coupled to the rails R through loop 12 and transmitted tothe second or right end of the track circuit.

A receiver R, responsive to the shifted frequency drives a return shuntrelay 20. The receiver R, consists of a filter 21 which responds to thefrequency F,+ and an amplifier 22 which is used to increase the gain ofthe received signal and drive a flip-flop 23 one output of which iscoupled to the return shunt 20. As in FlG. 2, the return shunt actuatesa contact 16 coupled in series with a resistor 17 across the rails R.This periodic shunting in accordance with the receiver R, causes anincrease in the track current. Track current for the second end isprovided by a second transmitter T feeding the rails throughtransmitting loop 25.

Transmitter T, consists of an oscillator 26 generating a frequency F,and an amplifier 27 for increasing the gain of the oscillator signal andcoupling the second carrier F to the rails R via the transmitting loop25. When the return shunt 20 is coupled across the rails, the currentnecessarily increases and a receiver R, responsive thereto drives thetransmitter T into a shifted frequency F The receiver R consists of afilter 28 responsive to the carrier frequency and a detector 29 capableof detecting a preset current threshold for providing a signal to thetransmitter T When the receiver R provides the shift signal to thetransmitter T the shifted frequency signal is coupled to the railsthrough the loop coil and transmitted back to the left end of the trackcircuit. A receiver R, is responsive to the shifted frequency 5+ andprovides an output signal to flip-flop 30. The receiver R, consists of afilter 31 and a driving amplifier 32 for providing an output signal tothe flip-flop 30.

The pulse generator 13 is coupled to output logic circuitry 40, as isthe R For each positive-going pulse of the generator 13, the test shunt14 is activated which causes an increase in track current and shift-upof frequency oscillator 10 with a subsequent reaction at the right endof the track circuit and a return test shunt, which is in turn receivedby the receiver R,'. The inputs to the output logic 40 must occur with acertain predetermined phase relation one to the other. If, for anyreason, the return test shunt is interrupted as by a failure of theapparatus or a vehicle shunt, the output logic 40 will provide anindication of such a condition by deenergizing the output relay 41 anddropping its associated front contact 42. A train in the center sectionof the track circuit, about 500 feet from each termination shunt S willcompletely interrupt communication from one end of the track circuit tothe other. However, when the train approaches the vicinity of the endzone; that is, the last 500 feet before the termination shunt S at somepredetermined current value the receiver R, or R, depending upon thedirection of travel of the vehicle will be activated by an increase inthe current. As long as the train is present in that end zone, thecurrent in the track circuit remains above that threshold and thereceiver R, or R is always activated. The period rate of introducing thetest shunt 14 to the circuit by pulse generator 13 is thereforesubstantially bypassed because the transmitter R, is always in theshift-up position. Under such conditions, the output logic 40 senses acessation of the inputs at required periodic intervals and causes therelay 41 to drop its contact 42. The logic 40 as previously noted inresponse for holding the relay 41 energized only wne its two inputs aresatisfied. By holding the transmitter frequency T, in the shift-upcondition, the apparatus at the right end of the track circuit isincapable of repeating the shift.

Pulses from the generator 13 coupled through transformer 43 alternatelyactivate the relays 44 and 45 and their associated contacts 46 and 47respectively. Diodes 48 and 49 assure that only pulses of the properpolarity actuate the associated relay. The contacts 46 and 47 arecoupled to AND gate 50 to alternate sets of associated inputs. Thesecond input to the output logic 40 is provided from the output offlip-flop 30 through transformer 51 for driving the relays 52 and 53 andtheir associated contacts 54 and 55 respectively. The diodes 56 and 57permit activation of the associated relays 52 and 53 only upon properpolarity as provided through the transformer 51 from the flip-flop 30.The contacts 54 and 55 are also coupled to the AND gate 50 to alternateinputs and as can be seen from the drawing, the contacts 46, 54 and 47,55 are associated inputs to the AND gate 50. Only when two associatedinputs to the AND gate 50 are energized simultaneously and the other twoinputs are deenergized will an output occur from the AND gate 50 forsetting and resetting the flip-flop 57. Alternate energization of theflip-flops inputs for setting and resetting activate a driver 58 whichproduces pulses to the relay 41 for maintaining the front contact 42closed. Capacitor 59 maintains the relay 41 during successive pulsesfrom the relay driver 58. If the period of the pulse generator 13becomes out of correspondence with the period of activation of thereturn shunt 20 and the associated shift frequency F as received by thereceiver R then the output logic 40 will cease to produce energy fordriving the driver 58 and maintaining relay 41. For each positive-goingpulse of pulse generator 13, there should be a single pulse from theflip-flop 30 on the associated output. Such a positive-going pulse fromthe generator 13 and the flip-flop 30 will cause the associated relays44 and 52 to close contacts 46 and 54 respectively satisfying theassociated inputs to the AND gate 50. Similarly, a negative-going pulseof the generator 13 and a negative output from the flip-flop 30 willcause the relays 45 and 33 to close the contacts 47 and 55 respectivelysatisfying the associated inputs to the AND gate 50 for resetting theflip-flop 57. Unless therefore the signal output of the transmitter Tprovided by the activation of the return test shunt 20 is received byreceiver R in a certain phase relation to the pulse generator 13, nooutput occurs from the output logic 40.

Any condition such as vehicle presence which would cause either of thereceivers R, or R to provide a continuous shiftup signal will cause acessation of this phased relation of the carrier F and the period returnof F If it were desired to increase the length of the track circuit, arepeater station near the mid-point of the track circuit may beinstalled and substantially double the length of the circuit. Therepeater circuit is coupled to the rails R at the mid-point as shown inFIG. 3 and a receiver R including filter 60 and amplifier 61 drivefrequency detector 62 which drives one input of transmitter T The secondreceiver R responsive to F includes a filter 63 and amplifier 64 drivingfrequency detector 65 which has an output which drives a second input totransmitter T Frequency detectors 62 and 65 are connected to oscillatorsin the transmitter T one of which is a shift-up oscillator 66 and theother is a shift-down oscillator 67. The receivers at opposite ends ofthe track circuit shown in FIG. 1 corresponding to R, and R areresponsive when used in connection with the repeater station to the Fand F signals respectively. The same continuity of indication isprovided using the repeater station. However, an extra frequency shifttransmitter T is necessary and variation in the receiver circuitsnecessary for receiving the particular mid-station signals are provided.This modification provides for nearly double the track length with aminimal amount of additional apparatus.

Code communications may be incorporated into this track circuit byutilizing an encoder 69. By coupling the encoder 69 to oscillator 26, ashift-down signal may be provided which is illustrated in the waveformdiagram of FIG. 4. The center frequency or carrier F, and a shift-up tothe F signal provides the vehicle detection information as previouslydescribed. The shift from F to F however, can be utilized to providecoded information during the times when the test shunt l4 and returntest shunt are not activated by initiating the shift-up phase of thevehicle detection. If, for example, as is contemplated in the presentinvention, a test shunt is introduced every second or some shortinterval of time, perhaps 50 milliseconds, then the remainder of the onesecond interval is available for transmission of coded information.Under these conditions, therefore, it is possible to provide not onlyvehicle detection information but also the cab signaling and 4 otheruseful indications without the necessity of providing wayside linewires.

Decoding is accomplished by coupling decoder 68 to one input of theoutput logic 40 associated with flip-flop 30. Information from encoder69 may be supplied by some program or other external source such as forexample a hot box detector. Decoder 68 output may be coupled to a signalon other warning devices to be actuated in accordance with theinformation supplied by encoder 69. The above is merely an example andit is obvious that many purposes may be seen by such a communicationschannel.

There has therefore been provided a system which incorporates theeconomical features of continuous rail track cir cuits with accurate endzone detection. There has also been provided a system which can bereadily adapted to provide for a continuous signaling from one waysideto the train without the necessity of wayside line communicationchannels.

While there has been described what is at present considered to be thepreferred embodiment of the present invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade without departing from the invention, and it is therefore aimed inthe appended claims to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:

l. A continuous rail track circuit apparatus having termination shuntsat each end for detecting the presence of a railroad vehicle shuntacross the rails indicative of vehicle presence comprising:

a. shunt means near one end of the circuit for periodically introducinga test shunt across the tracks;

b. receiver means coupled to the rails near the other end of the circuitsensitive to change in circuit parameters occasioned by said test shuntfor respondingly introducing a reply test shunt in accordance with theperiod of the shunt means; and

c. means at the first end of the circuit sensitive to change in circuitparameters occasioned by said reply test shunt for providing indicationof vehicle presence when said change in circuit parameters ceases.

2. The track circuit apparatus of claim 1 wherein the shunt meansincludes:

a. driver means at the first end for shunting the rails in the vicinityof the termination shunt;

b. transmitter means coupled to the rails at the first end for feedingthe rails with a carrier signal; and

c. shift means responsive to an increase in track current occasioned bythe test shunt for shifting the frequency carrier signal.

3. The track circuit apparatus of claim 2 wherein said test shunt meanscomprises:

a. an impedance coupled to a rail calibrated in accordance with apreselected value; and

b. switch means serially connected with said impedance operative whenactuated by said drive means for shunting said track with the calibratedimpedance.

4. The track circuit apparatus of claim 3 wherein said driver meansincludes a pulse generator for providing periodic pulses of energy foractuating said switch means.

5. The track circuit apparatus of claim 2 wherein said receiver meansincludes:

a. reply test shunt means at the second end for shunting the rails inthe vicinity of the termination shunt;

b. frequency responsive means coupled to the rails at the second end forintroducing the reply test shunt in response to the shifted carrierfrequency;

c. second transmitter means at the second end for feeding the circuitwith a second carrier signal; and

d. second shift means responsive to an increase in track currentoccasioned by the reply test shunt for shifting the frequency of thetransmitted signal.

6. The track circuit apparatus of claim 3 wherein the means at the firstend sensitive to the change in circuit parameters includes:

a. a capture receiver coupled to the rails responsive for generating asignal in accordance with the occurrence of the second frequency shiftcarrier; and

b. output logic means responsive to the period of the first shunt meansand the capture receiver signal for providing an output signal as longas the period of the first shunt means agrees with the occurrence of thesecond shifted carrier signal, said output indication of a clear trackcircuit and ceasing the output upon occurrence of disagreement of thesignals thereby providing indication of vehicle presence.

7. The track circuit of claim 4 wherein said output logic meanscomprises:

a. a switching means for changing conductance state in accordance witheach state of said capture receiver and said driver means;

b. gate means responsive to positive and negative conductance states ofthe capture receiver and driver means respectively for generating anoutput when said conductance states are in phase correspondence andclearing said output for indicating vehicle presence when saidcorrespondence ceases, thereby providing synchronism of the ends of thetrack circuits such that spurious signals are substantially ineffectiveto actuate said output logic means to an unsafe condition.

8. A continuous rail track circuit apparatus having termination shuntsat each end for detecting the presence of a railroad vehicle shuntacross the rails indicative of vehicle presence comprising:

a. train simulation means near one end of the circuit for periodicallyintroducing a fixed impedance across the rails commensurate with anideal vehicle shunt at a selected distance from the end of the trackcircuit,

tion shunt for providing indication of vehicle presence when said changein circuit parameters ceases.

9. The track circuit apparatus of claim 8 wherein said fxed impedance isa value indicative of a maximum allowable train shunt, typically on theorder of 0.6 ohm.

1. A continuous rail track circuit apparatus having termination shuntsat each end for detecting the presence of a railroad vehicle shuntacross the rails indicative of vehicle presence comprising: a. shuntmeans near one end of the circuit for periodically introducing a testshunt across the tracks; b. receiver means coupled to the rails near theother end of the circuit sensitive to change in circuit parametersoccasioned by said test shunt for respondingly introducing a reply testshunt in accordance with the period of the shunt means; and c. means atthe first end of the circuit sensitive to change in circuit parametersoccasioned by said reply test shunt for providing indication of vehiclepresence when said change in circuit parameters ceases.
 2. The trackcircuit apparatus of claim 1 wherein the shunt means includes: a. drivermeans at the first end for shunting the rails in the vicinity of thetermination shunt; b. transmitter means coupled to the rails at thefirst end for feeding the rails with a carrier signal; and c. shiftmeans responsive to an increase in track current occasioned by the testshunt for shifting the frequency carrier signal.
 3. The track circuitapparatus of claim 2 wherein said test shunt means comprises: a. anImpedance coupled to a rail calibrated in accordance with a preselectedvalue; and b. switch means serially connected with said impedanceoperative when actuated by said drive means for shunting said track withthe calibrated impedance.
 4. The track circuit apparatus of claim 3wherein said driver means includes a pulse generator for providingperiodic pulses of energy for actuating said switch means.
 5. The trackcircuit apparatus of claim 2 wherein said receiver means includes: a.reply test shunt means at the second end for shunting the rails in thevicinity of the termination shunt; b. frequency responsive means coupledto the rails at the second end for introducing the reply test shunt inresponse to the shifted carrier frequency; c. second transmitter meansat the second end for feeding the circuit with a second carrier signal;and d. second shift means responsive to an increase in track currentoccasioned by the reply test shunt for shifting the frequency of thetransmitted signal.
 6. The track circuit apparatus of claim 3 whereinthe means at the first end sensitive to the change in circuit parametersincludes: a. a capture receiver coupled to the rails responsive forgenerating a signal in accordance with the occurrence of the secondfrequency shift carrier; and b. output logic means responsive to theperiod of the first shunt means and the capture receiver signal forproviding an output signal as long as the period of the first shuntmeans agrees with the occurrence of the second shifted carrier signal,said output indication of a clear track circuit and ceasing the outputupon occurrence of disagreement of the signals thereby providingindication of vehicle presence.
 7. The track circuit of claim 4 whereinsaid output logic means comprises: a. a switching means for changingconductance state in accordance with each state of said capture receiverand said driver means; b. gate means responsive to positive and negativeconductance states of the capture receiver and driver means respectivelyfor generating an output when said conductance states are in phasecorrespondence and clearing said output for indicating vehicle presencewhen said correspondence ceases, thereby providing synchronism of theends of the track circuits such that spurious signals are substantiallyineffective to actuate said output logic means to an unsafe condition.8. A continuous rail track circuit apparatus having termination shuntsat each end for detecting the presence of a railroad vehicle shuntacross the rails indicative of vehicle presence comprising: a. trainsimulation means near one end of the circuit for periodicallyintroducing a fixed impedance across the rails commensurate with anideal vehicle shunt at a selected distance from the end of the trackcircuit, b. receiver means coupled to the rails near the other end ofthe circuit sensitive to change in circuit parameters occasioned by saidtest shunt for respondingly introducing a reply train simulation shuntin accordance with the period of the train simulator means, c. means atthe first end of the circuit sensitive to change in circuit parametersoccasioned by said reply train simulation shunt for providing indicationof vehicle presence when said change in circuit parameters ceases. 9.The track circuit apparatus of claim 8 wherein said fixed impedance is avalue indicative of a maximum allowable train shunt, typically on theorder of 0.6 ohm.